The start-up behavior of elevators is an essential criterion for the subjective judging of the feeling of the occupant, which in the start-up phase is determined basically by the acceleration, as well as by the acceleration changes and eventual vibrations. In this case, every acceleration of the elevator car and thus that of the passengers results from the superposition of the forces acting in the elevator system according to the formula Force equals Mass x Acceleration (F=MA). To be considered for the start-up in this connection are: A. the force of imbalance resulting from the difference between the car weight and the counterweight, B. the braking force of the blocking brake, C. the friction force resulting from the friction resistances of the movable parts, as well as D. the motor driving force resulting from the starting torque of the hoisting motor. As is generally known, there results during the start-up phase in some of these forces discontinuities in the derivative with respect to time. This relates A. to the braking force, because this force becomes suddenly zero on easing the mechanical blocking brake, as well as B. the friction resistances of all movable masses and the transmission components at standstill which are considerably greater than during movement and thus a very sudden change occurs on start-up from standstill. These mechanical discontinuities take place too rapidly to be controlled with the normal drive control. On the contrary, they cause control technological discontinuities and act according to the formula F=MA on the acceleration, which leads to strong changes in the acceleration, leading to jerks. Elevators of all types of construction tend to generate a start-up jerk when starting up from standstill.
In the past, a multitude of devices were proposed in order to eliminate this disagreeable start-up jerk completely or partially, and thereby to improve the comfort of travel. See, for example, German Publication No. 31 240 8. See also U.S. Pat. No. 4,828,075, "Elevator Drive Control Apparatus for Smooth Start-up;" and U.S. Pat. No. 5,076,399, "Elevator Start Control Technique for Reduced Start Jerk and Acceleration Overshoot," assigned to the same assignee as the present invention.
One cause for the start-up jerk is the unsteady derivative with respect to time of the friction during the transition from static friction to sliding friction as an elevator starts to move.